1. Field of the Invention
The invention relates to a method for determining the fill level of a medium according to the transit time method, wherein at least one transmission signal is transmitted, wherein at least one return signal is received, and wherein the return signal is evaluated at least in view of the process variable “fill level”.
2. Description of Related Art
When it is said that the method is intended for determining the fill level, and that the return signal is evaluated in view of the fill level, then such methods are meant, in which the fill level is only monitored, i.e., in which no continuous determinability of all fill levels must be present. In the same manner, the evaluation of the fill level does not only include displaying the level, this means that the return signal is processed with regard to the fill level, for example, is made available as a current or voltage signal for further processing. The transmission signals and the received signals are typically, in particular, electromagnetic signals, but of course, other signals are suitable, for example, sound waves.
Furthermore, the invention relates to a corresponding device for determining the fill level of a medium having at least one transmitting unit for transmitting at least one—in particular, electromagnetic—transmission signal, having at least one receiving unit for receiving at least one—in particular, electromagnetic—received signal, and having at least one evaluating unit for evaluating the received signal.
In industrial measurement technology, radar level measuring devices are frequently used to determine the level of media, such as liquids, bulk solids or sludge inside of containers, such as tanks, silos or tubes. The transit time method carried out by the measuring device is based on the physical law that the transit distance, e.g., of an electromagnetic signal is equal to the product of the transit time and propagation velocity. If the fill level of a medium in a container is measured, the transit distance corresponds to twice the distance between an antenna emitting the electromagnetic signal and receiving it again and the surface of the medium. The wanted echo signal—i.e., the signal reflected on the surface of the medium—and its transit time are mostly determined based on the so-called echo function or, respectively, digitized envelope curve. The envelope curve represents the amplitudes of the echo signals as a function of the distance “antenna—surface of the medium” or the transit time of the signal. The fill level can be calculated from the difference between the known distance of the antenna to the base of the container and the distance of the surface of the medium to the antenna as determined by measurement. The transmitted and received electromagnetic signals are mostly microwave radiation. Measurements using ultrasonic waves are also possible.
In practice, the return signal contains, unfortunately, not only of the actual echo signal reflected on the surface of the medium, but also interfering signals, for example, of multiple reflections resulting after partial penetration of the signal into a medium—in particular, a very dry one having low electric conductivity—or from reflections on the container walls, welding seams, recesses in the wall, mixers, filling lines, or the like. Material cones generally cause diffuse and directional backscatter derived from both the surface and angle of repose of the material cone itself. These interfering signals complicate the evaluation of the return signal and can lead to measurement errors. Great difficulties occur particularly in media having a small dielectric constant and low electrical conductivity.
German Patent Application DE 10 2009 055 262 A1 describes a method for determining the fill level using received signals with interfering signals. Comparison signals can be determined from the expected response signals in a learning phase as a result of application- and device-based test signals. In an operating phase, the received signals are compared with the reference signals, and when a deviation above a threshold value of the received signals occurs, the level is determined. In DE 10 2009 055 262 A1, other methods for evaluation of such received signals of the prior art having interference are also discussed.
The disadvantage of the prior art is that, in most cases, only methods that act selectively and are generally complex are provided for signal evaluation.